Added memcached to heroku demo for greater perfromance

Author: damianlegawiec

app.json

@@ -12,7 +12,8 @@
   "addons": [
     "heroku-postgresql:hobby-dev",
     "heroku-redis:hobby-dev",
-    "sendgrid:starter"
+    "sendgrid:starter",
+    "memcachedcloud:30"
   ],
   "scripts": {
     "postdeploy": "bundle exec rake db:migrate && bundle exec rake db:seed && SKIP_SAMPLE_IMAGES=true bundle exec rake spree_sample:load"